JARS v48n2 - Petal Blight in the Pacific Northwest

Petal Blight in the Pacific Northwest
E. White Smith
Tacoma, Washington

I think that I have petal blight on my rhododendron flowers and am not happy about it. I have seen this in years past and didn't recognize it as a problem to be concerned about. The spring of 1993 was warm and wet so that the flowers were affected much faster and more severely than in the past. To see the plants flower and to enjoy the color in the landscape is why I grow them, and if the flowers are destroyed quickly, the effect is really spoiled for me.
The petal blight I am talking about is Ovulinia petal blight or Ovulinia azaleae Weiss. It was first reported in South Carolina in 1931 and has spread around the world since then. My technical information says that this blight will also affect mountain laurel ( Kalmia latifolia ), but I didn't see this happen in my landscape. From my observations, I believe that the blight affects all rhododendrons including azaleas and the tropical vireyas. As summer arrived and the weather got drier the problem stopped even when the flowering plants were watered from overhead sprinklers, which seems odd to me.
As I write this article in mid September, I have quite a few vireyas blooming and a few hardy rhodies in flower and no blight signs are evident. Ovulinia azaleae is a fungus disease, which develops during moist weather at flowering times. You will notice small rust colored spots on the flower petals. These spots enlarge rapidly and the infected flowers become soft and watery. The entire flower head can become spoiled in a couple of days. I really suspected something was going wrong when I noticed that the lower (or first opening) flowers in a truss were spoiled and that the as yet unopened flowers were also affected and never opened at all. That is just not at all typical of rhododendrons. Even if the first flowers out in a truss get damaged somehow, the later flowers higher up in a truss will normally be fine. But not with this flower blightthey all get it. All of the flowers get slimy and limp.

Petal blight damage.
Petal blight damage.
Photo by E. White Smith

I got out a reference booklet and looked up petal blight and the pictures in the book sure looked like what I had. I then went out into the garden and looked for the hard black resting structures of the disease on the flowers and, sure enough, there were some. The other thing that happened to my plants was that the destroyed flowers tended to stick to the foliage as if they were glued on. The literature says to remove all of the diseased flowers from the garden. The petal blight disease will over-winter on the ground so you must try to get all of the spoiled flowers removed.
At first I thought I might not have the problem and I started to look around. Many rhodie gardens I saw last spring had this blight. Some of the gardens in my neighborhood also had the problem. I also noticed non-infected plants in full bloom throughout the area. These non-infected plants were separated from infected plants by quite a distance.
At our annual May potluck dinner meeting in the Rhododendron Garden at Point Defiance Park, I found flowers that looked to me as if they had petal blight. The Grays Harbor Chapter members in Aberdeen, Wash., have been telling me for years that they have petal blight and that it shows up in their annual truss shows. Because Aberdeen is right on the Washington coast they get a lot of wet spring days. When I was in Australia in 1988 I was shown petal blight by a friend who lives in the foothills west of Sidney. He was quite upset with the disease because it ruined his vireya flowers as fast as they opened.
So, I think I have petal blight and I don't like the idea at all. I am going to treat the plants for the problem, because I really like rhododendrons and the people who grow them. I am not going to give up. Still, I am not going to spray everything in sight, but I shall keep a small one-quart hand sprayer handy to use when necessary. Remember, a good pesticide application is to use the pesticide only when and where it is needed. Mix up only small batches and spray only where needed. I wrote to my good friend Bill Johnson who lives in Atlanta, Ga., and he answered saying that gardeners there do have petal blight and sent along a publication from the University of North Carolina at Raleigh. This publication described my problem to a T. It suggested spraying with "Bayleton 25% WP" at 1 to 2 teaspoons per gallon of water, or "Benlate* 50% WP" at 1 teaspoon per gallon. This spraying will need to be done every few days to control the problem.
My conclusions: If our rhododendron flowers melt away quickly during warm, wet springs, you should suspect that you might have petal blight and that you can do something about it. The first thing that you need to do is to go out and have a very close look at your flowers. Look for spots on the flowers that have turned light brown. Touch the flowers with your fingers to see if the brown spots are wet and slimy. If you have these conditions, then in a few days the entire truss should be affected and probably all of the trusses on the plant also. After about two weeks you might be able to find the small (2 to 8 mm wide) black sclerotia on the old flowers. These seem to be the over-wintering bodies of the disease. As quickly as possible you should remove the affected flowers and destroy them. Do not put these diseased items into your compost.

Compendium of Rhododendron and Azalea Diseases, American Phytopathological Society.
Plant Pathology Information Note # 232, Univ. of North Carolina at Raleigh.

Editor's Note: The DuPont product Benlate is not recommended for home gardeners.